The present invention relates to a digital modulator/demodulator (modem) for use with, for example, a facsimile apparatus which transmits picture data, symbol data and the like over a telephone line or like analog transmission line. More particularly, the present invention is concerned with a digital modem which achieves various functions necessary for a modem by use of a digital signal processor.
There has been proposed a digital modem of the type which uses a digital signal processor to realize various functions necessary for a modem such as coding, decoding, modulation, demodulation, line equalization and filtering. In U.S. Pat. No. 4,085,449, for example, such modem functions are achieved by means of a single processing unit. The problem encountered with this type of modem is that, since some of the functions need only to proceed at an encoding rate and some have to proceed at a sampling rate and, moreover, a relatively large amount of data are exchanged between the functions, the various functions cannot be implemented unless the single processor features a significant performance or a prohibitively intricate interface to the outside is employed.
One of the functions assigned to a modem is a shaping filter function. The tap constant of the filter depends on the sampling rate and modulation rate of signals. In a modem of the type employing a digital signal processor, since the sampling rate of signals is usually fixed, a system capable of using various modulation rates is unattainable unless it is furnished with a plurality of sets of tap constants which match with the desired modulation rates. To store the plurality of tap constant sets as data, a random access memory (RAM) or a read only memory (ROM) is required to have a store area with a substantial capacity. Further, data for realizing the filtering function, equalizing function or the like are stored in a data memory such as a RAM or a ROM. A prior art digital signal processor has been constructed to use the entire regions of the data memory for the respective modem functions. To access such regions, therefore, a substantial number of bits are required for an address portion of an instruction and a substantial number of address lines have to be installed, resulting in the need for a ROM having a large capacity.
Meanwhile, a facsimile apparatus operable in a so-called GIII mode has recently been standardized by CCITT (Consultive Committee of International Telegraph and Telephone) in order to promote fast transmission of picture information, and such apparatuses are now in practical use. In a GIII facsimile apparatus, it is a usual practice to transmit by a digital modem a coded signal with the redundancy of a facsimile picture signal cut down by a digital band compression technique, with a view to shortening the transmission time. Such a modem comprises, in accordance with the GIII standard, modems prescribed by the CCITT Recommendations V.27 and V.29. The V.27 modem operates with a transmission rate of 4,800 bps (bits/second) and an 8-level differential phase-shift keying (PSK) system, while the V.29 modem operates with a transmission rate of 9,600 bps and a quadrature amplitude modulation (QAM) system.
To date, in order that a received signal on a transmission line may be surely recovered, a digital signal processor for performing modulation and computation processings is designed to handle as large a number of data bits as possible (e.g. 16 bits). Therefore, it is a prerequisite to convert analog signals on a transmission line to digital signals which are identical in the number of bits with data handled by the digital signal processor. However, an analog-to-digital converter accommodating a relatively large number of data bits and operable with a conversion time short enough for relatively high-speed data transmission is expensive, resulting in an expensive digital modem construction.